 (defn ^boolean is-cell-empty?
   "Indicates whether a cell is owned by any player"
   [cell]
-  (or (= cell :empty) (= cell :wall)))
+  (or (nil? cell) (= cell :empty) (= cell :wall)))
 
 (defn available-cells-by-dir
   "Indicates the convertible cells when clicking at [x y], and returns:

         y (+ yi dy)
         looser nil
         taken []]
    (if-let [c (get-in board [x y])]
      (cond
        (is-cell-empty? c) nil ;; No move: reached end and only 1 type of cell
        (and looser (not= looser c)) {:winner c      ;; Who wins the cells

                (+ x dx) (+ y dy)
                (or looser c) 
                (conj taken [x y])))
      )))

(defn available-moves-at
  "Provides the list of moves that can be done from a cell"
